The present invention relates to an image forming apparatus and a developing device disposed in the image forming apparatus.
In a conventional image forming apparatus of an electro-photography type, a developing device is provided with a toner supplying roller made of a silicone material such as a silicone rubber and the like (refer to Patent Reference). Accordingly, it is possible to press the toner supplying roller against a developing roller with an increased pressing force at an abutting portion (a nip portion) thereof, so that a large amount of toner can be physically contacted with a surface of the developing roller. As a result, it is possible to obtain a toner layer with a large thickness on the developing roller, thereby preventing a density variation (a blurred image).    Patent Reference Japanese Patent Publication No. 07-333996
In the conventional image forming apparatus, in order to increase an operational speed of the developing device or decrease a load torque thereof, when the toner supplying roller is made of a urethane material with a low hardness, it is difficult to physically contact a large amount of toner with the developing roller. In this case, it is necessary to stably supply an electric field to the nip portion. Further, it is necessary to form the developing roller using a material with a fast response to the external electric field and a small dielectric constant, that is, a small remaining charge property. Accordingly, it is possible to form an image without the density variation (the blurred image).
In the conventional developing device described above, however, when the developing roller is formed of a material with the small remaining charge property, the developing roller tends to exhibit an effect of releasing electric charges of charged toner. Accordingly, when the developing roller has a remaining charge property better than a target level, a fog tends to occur on a white sheet. As a result, it is necessary to precisely control the remaining charge property of the developing roller within a specific desired range, and it is difficult to constantly balance an acceptable performance with respect to the density variation and the fog.
In view of the problems described above, an object of the present invention is to provide a developing device and an image forming apparatus capable of solving the problems of the conventional developing device. In the present invention, a developer supplying member includes a foamed member having an electrical resistivity between 1.9×104 and 7.6×107Ω. Accordingly, it is possible to achieve a good performance with respect to the density variation and the fog.
Further objects and advantages of the invention will be apparent from the following description of the invention.